Dalton’s law states that the total pressure in a closed system containing a mixture of gases is equal to the sum of the partial pressures of each of the individual gases.
The gas molecules are so far apart from one another that each acts independently.
There is presumably very little oxygen in the head space of a bottle of beer. So the oxygen partial pressure is low. The primary partial pressure in the bottle is from CO2.
The amount of oxygen in open air leads to about 3 psi partial pressure of oxygen outside the bottle.
So, the oxygen partial pressure outside the bottle is higher than the oxygen partial pressure in headspace of the bottle.
This net decrease in pressure from outside to inside the bottle leads oxygen to want to get into the bottle. The cap forms a seal to minimize oxygen ingress (and CO2 escape) and some caps are designed to absorb oxygen to limit the damage. But, apparently cans have a better seal than bottles.
And the tl:dr version is that the pressure of CO2 is independent of the pressure of oxygen. There is very, very little pressure of oxygen inside the bottle, and very large pressure of oxygen outside the bottle. The cap or can seal is not 100% impermeable. Oxygen will slowly diffuse through over time.
And while CO2 will diffuse out of the bottle as well, you will notice the effects of a small amount of extra O2 sooner than a small amount of lower CO2
That is fascinating indeed. So, If I understand, because there are only a few molecules of oxygen inside the bottle and a greater quantity of oxygen in atmospheric pressure, the pressure outside (atmospheric) will attempt equalize the "like" gas inside the bottle? And, likewise, CO2
inside the bottle will attempt to equalize with "like" gas in atmospheric? That is fascinating information I have never heard or read about. I'm going to read into Dalton's Law.
I'm thinking Die Beerery wasn't able to better explain this complex principle because he simply couldn't!
Well done tommymorris & erockrph! Thank you!